I am detecting Influenza protein (NP) in a set of samples. I would like to have a control +ve for my WB. Now I thought to clone the NP gene in a plasmid that express it. But how to show the protein of the NP in WB ?

If you can express if off a plasmid and can get (or make) an antibody against the protein, that you can then show detects a band of the correct size and can be competed by peptide, then that is usually considered sufficient.